Can't read the article - it's been trying to load for the past 5 minutes...
My first reaction - that idea has been cooked up by somebody who thinks that farming/crofting is all about tinkering about lightly in the beautiful countryside with healthy fresh air etc! (Not having read the article, I might be wrong.)
Work on farm, as you have pointed out, is always bound up with an element of danger. I do some work on farms, and all of it means hard physical labour, and usually thinking quickly, taking decisions on the job etc., and all that for not much pay. Vulnerable people would have to be supervised closely; it can work in a "special" environment, where other staff are trained to work with vulnerable people - but that would mean extra cost, extra staff, and no real help for the farmer/crofter, no matter how enjoyable it could be for the vulnerable person involved. Yes, fine for little jobs like collecting eggs, at certain times - but would the necessary input (i.e. taking somebody to your place) be justified by the outcome? I doubt it.